This invention relates generally to an image forming apparatus which is applied to a copier, a printer, a facsimile machine and the like.
Conventionally, an image forming apparatus has been provided wherein a developing unit develops a toner image by electrostatically supplying toner charged with electricity to an electrostatic latent image on an electrostatic latent image carrier (photoreceptor), and the developed toner image is transferred onto a sheet of paper so as to form an image. In this kind of the image forming apparatus, the toner remaining on the electrostatic latent image carrier after transferring the image is collected by a cleaning unit, and then, conveyed and returned into the developing unit for recycling (hereinafter, this process is referred to as toner recycling, and the toner to be reused is referred to as recycled toner).
As to the image forming apparatus which carries out the above-mentioned toner recycling, there has been proposed an art for changing a recycled toner supplying rate (a ratio between recycling and disposal of the collected toner) according to each of copy modes, controlling image forming conditions, or controlling the toner supplying rate according to a fog toner amount on a photoreceptor drum detected by a sensor, thereby preventing the degradation of the image quality (e.g. refer to Japanese Patent Gazette No. 2668527, Japanese Patent Examined publications Nos. 5-59428 and 4-54955, and U.S. Pat. No. 5,604,575).
The above Japanese Patent Gazette No. 2668527 discloses an apparatus which comprises a toner collecting chamber for containing collected toner, a toner container connected to a developing chamber, a toner re-supplying means for conveying the collected toner of the toner collected chamber into the toner container, and adjusts image forming conditions according to the number of image forming, thereby controlling the degradation of the image caused by change in toner properties associated with usage. Further, the Japanese Patent Examined publication No. 5-59428 discloses an apparatus which measures toner image density which remains on a photoreceptor, and controls the amount of toner to be replenished from a recycled toner and toner replenishing device into a developing unit based on the measured density value. Furthermore, the Japanese Patent Examined publication No. 4-54955 discloses an apparatus which has a conveyance passage for conveying collected toner into a developing unit and a collecting container, and varies a ratio of the recycled toner by a valve located in the conveyance passage, thereby decreasing a reducing ratio to 0 or value less than usual when the number of copied pages is less than the predetermined continuous number.
The U.S. Pat. No. 5,604,575 discloses an apparatus which comprises a first conveyer mechanism for conveying collected toner into a container, and then into a disposal part, and comprises a second conveyer mechanism for conveying the toner into a replenishing means, and an AIDC (auto image density control) sensor for detecting image density on a photoreceptor, and which selects a predetermined fresh toner/collected toner replenishing amount according to the detected value, thereby controlling the conveyer mechanism. Further, the U.S. Pat. No. 5,604,575 discloses an apparatus which comprises a controller for controlling the supplying rates of recycled toner and fresh toner, wherein the controller controls a recycled toner ratio to all the toner according to output of an ATDC sensor, and controls a toner replenishing amount according to output of an AIDC sensor, thereby decreasing the recycled toner replenishing amount, and increasing the fresh toner replenishing amount (toner density (a ratio between toner and carriers) is not changed in developer) when the developing ability is lowered.
Moreover, there has been known an art for using air force in order to collect toner powder sprinkled by rotation of a developing sleeve of a developing unit. In this art, however, the collected toner powder remains in and fills a toner suction duct, which prevents a stable operation for collecting toner powder, thereby making it impossible to maintain performance of the suction duct. Consequently, when the suction duct is filled with the toner powder, a user has to get a service person to replace the suction duct. Thus, it involves a troublesome maintenance.
Furthermore, there has been provided an art wherein, if an apparatus which has a mechanism for conveying and collecting the waste toner (toner to be disposed of), detects toner conveying torque and excessive load during a copying operation, the apparatus disables further copying operations, otherwise, the apparatus disables after making copies of specified pages following the detection of the excessive load, thereby preventing the degradation of the image quality (e.g., refer to Japanese Patent Gazette No. 2642353).
However, as to the above-mentioned recycled toner, since its degraded property prevents toner from being sufficiently charged with electricity (the toner which is insufficiently charged will be hereinafter referred to as undercharged toner), besides the recycled toner contains the undercharged toner which remained on the electrostatic latent image carrier without being transferred onto a transferred material (paper and the like), or contains powder generating from the paper being conveyed (hereinafter this powder is referred to as paper powder). If the undercharged toner or paper powder is re-supplied into the developing unit, the function does not sufficiently effect, which may cause ground fogging and contamination in the apparatus. This problem is more likely to occur as the toner density becomes higher. Besides, since adopting lower toner density produces an image having a decreased density, charging potential, developing bias and the like are set to be high potential, but this causes a problem such as carrier depositing, degradation of the image quality, leaking.
Moreover, the toner containing the undercharged toner or paper powder has decreased fluidity. Then, the toner is deposited on a blade and the like composing a conveyer mechanism, which increases the amount of toner solidified particles. If a user continuously makes copies in this state, the solidified toner is solidified in the conveyance pipe. Then, the solidified toner particles are conveyed into the developing unit, so that this will bring a problem of generating an image noise such as white and black spots. In a state that the solidified particles are generated, the user has to replace developer. This replacement operation consumes time, which produces inconvenience of making impossible for the user to make a copy during that time.